In the process of acquiring oil and/or gas from a well, it is often necessary to stimulate the flow of hydrocarbons via hydraulic fracturing. The term “fracturing” refers to the method of pumping a fluid into a well until the pressure increases to a level that is sufficient to fracture the subterranean geological formations containing the entrapped materials. This process results in cracks and breaks that disrupt the underlying layer to allow the hydrocarbon product to be carried to the well bore at a significantly higher rate. Unless the pressure is maintained, however, the newly formed openings close. In order to open a path and maintain it, a propping agent or “proppant” is injected along with the hydraulic fluid to create the support needed to preserve the opening. As the fissure is formed, the proppants are delivered in a slurry where, upon release of the hydraulic pressure, the proppants form a pack or a prop that serves to hold open the fractures.
To accomplish the placement of the proppants inside the fracture, these particles are suspended in a fluid that is then pumped to its subterranean destination. To prevent the particles from settling, a high viscosity fluid is often required to suspend them. The viscosity of the fluid is typically managed by addition of synthetic or naturally-based polymers. There are three common types of polymer-enhanced fluid systems in general use for suspending and transporting proppants during hydraulic fracturing operations: slickwater, linear gel, and crosslinked gel.
In slickwater systems, an anionic or cationic polyacrylamide is typically added as a friction reducer additive, allowing maximum fluid flow with a minimum of pumping energy. Since the pumping energy requirements of hydraulic fracturing are high, on the order of 10,000-100,000 horsepower, a friction reducer is added to slickwater fluids to enable high pumping rates while avoiding the need for even higher pumping energy. While these polymers are effective as friction reducers, they are not highly effective as viscosifiers and suspending agents. Slickwater polymer solutions typically contain 0.5-2.0 gallons of friction reducer polymer per 1000 gallons of slickwater fluid, and the solutions have low viscosity, typically on the order of 3-15 cps. At this low viscosity, suspended proppant particles can readily settle out of suspension as soon as turbulent flow is stopped. For this reason, slickwater fluids are used in the fracturing stages that have either no proppant, proppant with small particle size, or low proppant loadings.
The second type of polymer enhanced fluid system is known as a linear gel system. Linear gel systems typically contain carbohydrate polymers such as guar, hydroxyethylcellulose, hydroxyethyl guar, hydroxypropyl guar, and hydroxypropylcellulose. These linear gel polymers are commonly added at a use rate of 10-50 pounds of polymer per 1000 gallons of linear gel fluid. These concentrations of linear gel polymer result in a fluid with improved proppant suspending characteristics vs. the slickwater fluid. The linear gel fluids are used to transport proppants, at loading levels of about 0.1 to 1 pound of proppant per gallon of fluid. Above this proppant loading level, a more viscous solution is typically required to make a stable suspension.
Crosslinked gel is the most viscous type of polymer-enhanced fluid used for transporting of proppant. In crosslinked gel systems, the linear gel fluid as described above is crosslinked with added reagents such as borate, zirconate, and titanate in the presence of alkali. Upon crosslinking of the linear gel fluid into a crosslinked gel fluid, the viscosity is much higher and the proppants can be effectively suspended. The linear gel and crosslinked gel fluids have certain advantages but they require a high dose rate of expensive polymer.
Modifications of proppant particles could be used advantageously to improve their performance in hydraulic fracturing systems. First, if the proppant particles were more buoyant, a less viscous suspension fluid could be used, which would still convey the particles to the target area but which would be easier to pump into the formation. Second, it is desirable that the proppants remain where they are placed throughout the lifetime of the well after they have been injected into a fracture line. If changes within the reservoir during well production force the proppants out of position, production equipment can be damaged, and the conductivity of the reservoir formation can be decreased as the reservoir pores are plugged by the displaced proppants. Third, the proppants in the system should be resistant to closure stress once they are placed in the fracture. Closure stresses can range from 1700 psi in certain shale gas wells, up to and exceeding 15,000 psi for deep, high temperature wells. Care must be taken that the proppants do not fail under this stress, lest they be crushed into fine particles that can migrate to undesirable locations within the well, thereby affecting production. Desirably, a proppant should resist diagenesis during fracture treatment. The high pressures and temperatures combine with the chemicals used in frac fluids can adversely affect the proppant particles, resulting in their diagenesis, which can eventually produce fine particulate matter that can scale out and decrease the productivity of the well over time.
Current proppant systems and polymer-enhanced fracturing fluids endeavor to address these concerns, so that the proppants can be carried by the fracturing fluids, can remain in place once they arrive at their target destination, and can resist the closure stresses in the formation. One approach to preparing suitable proppants includes coating the proppant materials with resins. A resin-coated proppant can be either fully cured or partially cured. The fully cured resin can provide crush resistance to the proppant substrate by helping to distribute stresses among the grain particles. A fully cured resin can furthermore help reduce fine migration by encapsulating the proppant particle. If initially partially cured, the resin may become fully cured once it is placed inside the fracture. This approach can yield the same benefits as the use of a resin that is fully-cured initially. Resins, though, can decrease the conductivity and permeability of the fracture, even as the proppants are holding it open. Also, resins can fail, so that their advantages are lost. Resin-based systems tend to be expensive and they are still prone to settling out of suspension.
In addition, there are health, safety and environmental concerns associated with the handling and processing of proppants. For example, fine particulates (“fines”), such as crystalline silica dust, are commonly found in naturally occurring sand deposits. These fines can be released as a respirable dust during the handling and processing of proppant sand. With chronic exposure, this dust can be harmful to workers, resulting in various inhalation-associated conditions such as silicosis, chronic obstructive pulmonary disease, lung cancers in the like. In addition to these health effects, the fines can cause “nuisance dust” problems such as fouling of equipment and contamination of the environment.
Another approach to preparing suitable proppants involves mixing additives with the proppant itself, such as fibers, elastomeric particles, and the like. The additives, though, can affect the rheological properties of the transport slurry, making it more difficult to deliver the proppants to the desired locations within the fracture. In addition, the use of additives can interfere with uniform placement of the proppant mixture into the fracture site. While there are known methods in the art for addressing the limitations of proppant systems, certain problems remain. There is thus a need in the art for improved proppant systems that allow precise placement, preserve fracture conductivity after placement, protect well production efficiency and equipment life, simplify hydraulic fracturing operations, reduce environmental impact, and promote worker health and safety. It is further desirable that such improved systems be cost-effective.